Previously, and for larger astronomical telescopes, these forces were taken up almost exclusively by load-relieving devices which supported the mirror at its outer edge where the gravity plane of the mirror intersects the outer periphery thereof. In the following, these forces will be referred to as radial weight components.
Examples of such edge supporting systems are described in published German patent applications DE-A-33 13 324, DE-A-29 03 805 and DE-A-24 14 266.
This type of radial load relief of the body of the mirror cannot be used without more if the diameter of the mirror exceeds a predetermined amount or for so-called segment mirrors whose individual segments are to be positioned one next to the other so that no space is available at its edges for the load-relieving devices.
Further, it is known to relieve the radial weight loads by systems which engage bores on the rear side of the mirror which penetrate up to the gravity plane of the mirror. Secondary mirrors of a 3.5 m telescope such as the MPIA on Calar Alto as well as the main mirror of the 5 m telescope on Mount Palomar are journalled in this manner for example.
The last-mentioned type of radial load relief is schematically shown in FIG. 1 of the drawing of this application. In FIG. 1, reference numeral 104 designates the bores which penetrate up to the gravity plane 106 of the mirror body 101. An end of a two-arm lever 105 engages in each bore. The lever 105 is pivotably journalled about an axis 108 on the mirror frame 109 and carries a counterweight 107 at its other end.
The disadvantage of this solution is that the bores on the rear side 103 of the mirror body cause disturbances in the elastic behavior which can bring about changes in the mirror surface 102. This effect is especially noticeable in thin mirrors. It is however not possible without more to do without the bores in the mirror body and to allow the radial load relief to act on the rear side of the mirror. This is true because when the mirror body is supported outside of its gravity plane, a disturbing moment develops which likewise causes form changes of the mirror surface.